The three patents described below relate to the interference cancellation techniques described in this application.
U.S. Pat. No. 3,699,444, INTERFERENCE CANCELLATION SYSTEM, describes a radar system circuit which uses a portion of the transmitted signal, after phase shifting and attenuation, to cancel the transmitted signal received at the receiver antenna.
U.S. Pat. No. 3,716,863, INSTRUMENT LANDING ERROR CORRECTING SYSTEM described apparatus for cancelling an interfering signal coherent with the desired signal, but varying in amplitude and phase.
U.S. Pat. No. 4,016,516, REFLECTIVE SIGNAL CONTROLLER, describes a circuit for varying the amplitude and polarity of an rf signal. Parts of said circuit may be of use in this inventive application.
None of the above references teaches or suggests a technique for cancelling interference based on a difference of polarization between it and the desired signal.
Described herein is apparatus for preventing the jamming of radar by an electronic countermeasure (ECM) jamming or interference signal; and more particularly, apparatus for receiving two or more different polarizations of the radar return signal and the interference signal, and using that different polarization received signal to cancel the interference signal without also cancelling the desired radar return signal.
It is common for radar systems to be jammed by hostile electronic countermeasure interference signal sources. These electronic countermeasure systems typically monitor the radar band, accurately determine the frequency, pulse repetition rate another radar system characteristics, and transmit interfering signals of sufficient power and of appropriate timing to render the radar inoperative. These countermeasure systems operate in real time so that a change of radar frequency will instantaneously be followed by a change of interference frequency. Therefore, some method of interference avoidance is required what will act to cancel interference, both pulsed and continuous, even at the exact frequency of the radar system.
The prior art includes U.S. Pat. No. 3,716,863, INSTRUMENT LANDING ERROR CORRECTION SYSTEM, commonly assigned, which cancels an interference signal by producing a correction signal of equal frequency and amplitude but of appropriate polarity. When the interference, correction and desired signals are received and summed, the interference signal is cancelled and the desired signal remains. This system is useful where the desired and interference signals are of the same frequency and are coherent but differ in phase. An example is an aircraft receiving an ILS signal directly from a transmitter and simultaneously receiving a reflected ILS signal from a nearby structure.
A system employing this principle may employ a circuit for varying the basic signal phase and amplitude to produce a correction signal. Such a circuit is described in U.S. Pat. No. 4,016,516, REFLECTIVE SIGNAL CONTROLLER, commonly assigned. This signal controller is designed to be inserted into the path between a source and the utilization device to allow the control of signal amplitude ratio and polarity.
The prior art thus recognizes the problem of interfering signals of the same frequency, and describes the generation of a correction signal of appropriate phase to cancel said interference. This is possible since the correction signal generator is coupled to, and therefore is coherent with the interference signal source.
In the case where the interference signal is produced by an electronic counter measure source (ECM), however, there can be no coupling to the source to generate a coherent correction signal, and cancellation of a jamming signal is not feasible by this method.
An alternative is to cancel such interference by taking advantage of the difference in signal polarization between desired and interference signals. This technique uses circuits equivalent to those required by prior cancellation systems, but requires a different antenna installation. In the case where the interference and desired signals do not have polarization differences, provision is made to change the polarization of the desired signal.
The proposed system comprises a receiver equipped with two receiving antenna ports, each configured to receive waves at a particular polarization, the two polarizations being ninety degrees out of phase with each other. For example, one could be configured to receive vertically (V) polarized waves and the other, horizontal (H). Then, to the extent that the ratios in the interference and radar return signals are different at the V and H receive ports, a cancellation signal can be obtained by adjusting the amplitude ratio and electrical phase angle of the V or H receive port signal that does not normally receive the desired signal. Therefore, cancellation of the interference signals only can be provided even if the desired and interference signals are at the same frequency.
This system can function wherever the desired versus interference ratios are not the same at the two polarization ports. This would be true if the interference were vertically polarized and the desired signals horizontally, for instance. Also, the interference may be circularly polarized and the desired signal, vertically. In the latter case, the horizontally polarized antenna would receive the interference while the vertically polarized antenna would receive both interference and desired signals. A signal received via one channel (port) could be used on the other channel for interference cancellation.
In the case where the interference signals polarization angle is matched to that of a desired radar signal, the radar polarization angle may be changed by the operator to avoid cancellation of the desired radar signals. In the case where the energy is radiated from a radar antenna, the polarization of the radiated signal could be rotated through the use of the appropriate waveguide xe2x80x9cplumbingxe2x80x9d. The system will then again operate as stated above.
One variation that the system has to compensate for is the ratio of received interference signal power received by each antenna port. For instance, circularly polarized interference has equal amounts of horizontally and vertically polarized signal power. Therefore, the same amount of interference power received at one antenna port has to be subtracted from the signal received at the other antenna port. On the other hand, horizontally polarized interference would provide a relatively small amount of interference at the vertical antenna port. This system variable is the amplitude ratio between the channels. The system automatically reduces (or increases) the amplitude ratio required to provide interference cancellation.
Another system variable is the amount of time or phase delay between channels. The same interference signal may be received by one receiving antenna port a fraction of a wavelength ahead of the other. To compensate, an electrical phase angle and/or a variable delay in the system is provided, so that cancellation signals are produced with the proper electrical phase angle to cancel the interference optimally.
Both the phase and amplitude ratio control circuits are monitored and, to the extent that perfect cancellation was not produced, error signals are generated. These error signals are then fed back to the phase and amplitude control circuits, closing the loops to make automatic system corrections.
Therefore, an object of this invention is to protect a radar receiver from interference by differentiating between normal returns and interference, based on differences between their polarizations and to cancel the interference thus detected.